1. Field of the Invention
The present invention relates to an image display apparatus that is worn on the user's head so as to be used in front of the user's eyes. More particularly, the present invention relates to an image display apparatus that allows simultaneous observation of a displayed image and of the outside world.
2. Description of Related Art
Head-worn image display apparatuses of the type worn on the head so as to be used in front of the eyes have been becoming increasingly popular as image display apparatuses for personal use. It is preferable that such image display apparatuses be made compact and lightweight.
For this reason, such image display apparatuses are typically built with a small display device in combination with an observation optical system that presents an enlarged virtual image of the image displayed on the display device. Moreover, it is preferable that such image display apparatuses be so designed that the distance from the eyes to the presented image (virtual image) is variable to allow the user to observe the image with a dioptric power that suits the user.
Head-worn image display apparatuses that allow the dioptric power to be varied are proposed, for example, in the patent publications cited below.                Patent Publication 1: Japanese Patent Application Laid-open No. 2001-264681        Patent Publication 2: Japanese Patent Application Laid-open No. 2000-330069        Patent Publication 3: Japanese Patent Application Laid-open No. 2000-298237        Patent Publication 4: Japanese Patent Application Laid-open No. H6-326945        
In the image display apparatus proposed in Japanese Patent Application Laid-open No. 2001-264681 (laid-open on Sep. 26, 2001), the dioptric power is varied by moving along the optical axis an optical element provided in an observation optical system.
In the image display apparatus proposed in Japanese Patent Application Laid-open No. 2000-330069 (laid-open on Nov. 30, 2000), a prism having a free-form surface is used as an observation optical system, and the dioptric power is varied by varying the distance between the prism and a display device.
In the image display apparatus proposed in Japanese Patent Application Laid-open No. 2000-298237 (laid-open on Oct. 24, 2000), a variable-shape mirror is used as part of an observation optical system, and the dioptric power is varied by varying the shape of the mirror.
In the image display apparatus proposed in Japanese Patent Application Laid-open No. H6-326945 (laid-open on Nov. 25, 1994), a liquid crystal lens of which the refractive index varies according to the voltage applied thereto is used as part of an observation optical system, and the dioptric power is varied by varying the refractive index of the liquid crystal lens.
In recent years, there have also been proposed so-called “see-through” image display apparatuses that allow simultaneous observation of an image and of the outside world. A see-through image display apparatus can be worn all the time in daily life, and thus allows the user to be in touch with information all the time. This makes see-through image display apparatuses useful. For example, during a lecture, the contents of the speech can be presented in the form of images so that hearing-impaired people can easily understand them.
The problem here is that the part on which the user focuses his or her attention in the outside world changes constantly. As the focus of the user's attention changes, the observation distance varies greatly. Thus, if the presented image is presented with a fixed dioptric power, the dioptric powers appropriate for the observation of the outside world and of the image tend to differ greatly from each other. This greatly strains the user's eyes.
For this reason, in see-through image display apparatuses, it is particularly important that an image be presented with a variable dioptric power. It is especially preferable that the dioptric power with which the image is presented be switched quickly as the observation distance changes.
Certainly it is possible to achieve a variable dioptric power in a see-through image display apparatus on the basis of the conventional techniques proposed in the patent publications mentioned above. Doing so, however, has the following disadvantages. With image display apparatuses like those proposed in Japanese Patent Applications Laid-open Nos. 2001-264681 and 2000-330069, where a particular optical element needs to be moved mechanically, it is necessary to provide a drive mechanism and a mechanical power source. This makes it difficult to realize a compact, lightweight image display apparatus.
With image display apparatuses like that proposed in Japanese Patent Application Laid-open No. 2000-298237, where a variable-shape mirror is used, it is necessary to provide an arithmetic processor, a drive circuit, and the like for controlling the shape of the mirror. This makes it difficult to realize a compact, lightweight image display apparatus.
With image display apparatuses like that proposed in Japanese Patent Application Laid-open No. H6-326945, where a liquid crystal lens is used, it is difficult to design a flat optical system, and in addition it is necessary to provide a controller for controlling the applied voltage. This makes it difficult to realize a compact, lightweight image display apparatus. Moreover, using a special and thus expensive optical element like a variable-shape mirror or a liquid crystal lens results in an accordingly expensive image display apparatus.